This invention relates to communication networks and, in particular, to switches in an asynchronous transfer mode (ATM) network.
In a communication network, units of data must be routed between different points of the network through switches. In ATM networks, traffic streams of mixed types of information are carried according to the concept of "bandwidth on demand." The data units, ATM cells, are transferred in accordance with the varying requirements of each ATM cell. Data units which are time critical are given priority in network routing. Data units which are information critical are given priority against cell loss. This ability to handle video, voice, computer data and other information has led to the wide acceptance of ATM as a prospective network standard.
An ATM network switch must route information reliably with minimum delay and loss, but the wide variation in requirements in ATM networks creates heavy demands upon the ATM switch. In particular, previous designs have placed a buffer at each output port of the ATM switch to store cells arriving faster than the output port can transmit to its output channel. The buffers created operational tolerance to differing demands on the ATM switch to avoid losing cells. A variation of this concept has been to place a storage buffer which is commonly used by all the output ports of the ATM switch.
These different designs have various shortcomings, including complexity, high cost, and impediments to configuration changes. Another shortcoming is that the simple buffering at the output port set a limit on the amount of buffering available to the output port. However, the probability of congestion at an output port is directly related to the number of input ports feeding the output port. As more input ports are added to the ATM switch, the probability of overwhelming an output buffer increases. For the design with a common buffer shared by all the output ports, performance is often not better. If enough data arrives at an output port, a large fraction of the common buffer is occupied by the cells destined for the output port and is unavailable to the other output ports. This can cause problems, particularly in networks with multiple servers.
The present invention solves or substantially mitigates these problems with an ATM switch which has buffering distributed for higher performance and which allows for greater modularization of design.